1. Technical Field
The present invention relates to an oscillation circuit exhibiting high frequency oscillation and an electronic device provided with the oscillation circuit.
2. Prior Art
FIG. 18 shows the structural principle of a feedback type oscillator. The feedback type oscillator using a piezoelectric resonator comprises a feedback circuit 101 that consists of a resonator, a phase shift circuit, and wiring for connecting the elements. Among them, the resonator primarily determines the oscillation frequency. An input voltage Vi is applied to the input. The amplifier 100 has a gain G that provides the output voltage Vo at the output that is G times larger than the input voltage Vi. This is expressed by the following equation:
Vo=Vixc2x7G. 
The output voltage Vo is fed back to the input of the amplifier 100 by xcex2Vo (not xe2x80x9czeroxe2x80x9d) via a feedback circuit 101 having a voltage feedback rate xcex2. The feedback voltage Vf is expressed by the following equation:                               V          f                =                  Vo          ·          β                                        =                  Vi          ·          G          ·                      β            .                              
When, Vf greater than Vi and both have the same phase, the positive feedback in which the feedback voltage is larger than the input voltage causes oscillation.
Assuming Vi has a phase xcex8i, Vf has a phase xcex8f, the amplifier G yields a phase shift xcex8G, and the feedback circuit 101 yields a phase shift xcex8xcex2, then
Vfxc2x7ejxcex8f=Vixc2x7Gxc2x7xcex2xc2x7ej(xcex8i+xcex8G+xcex8xcex2) greater than Viej(2xcfx80+xcex8i) 
To satisfy equation (1), the phases must be equal. Therefore,
xcex8G+xcex8xcex2=2xcex7xcfx80, (xcex7=0,1,2, . . . ) 
Gxc2x7xcex2 greater than 1 
The expression (2) is the phase requirement and the expression (3) is the amplitude requirement for the oscillator. Satisfying expressions (2) and (3), causes the feedback type oscillator in FIG. 18 to oscillate.
When the feedback voltage Vf increases high enough to saturate the output voltage Vo of the amplifier 100, this leads to the steady state and stabilized output. The amplitude requirement is then:
Gxc2x7xcex2=1. 
Piezoelectric oscillation circuits in prior art, employ an amplifier with a high operation performance compared to oscillation frequencies. Therefore, the phase shift rate due to time delay or phase delay can be neglected. The amplifier is considered to be a positive phase or an inverting amplifier for input signals.
The phase shift requirement for the oscillation condition is primarily directed by the phase shift requirement of the resonator and phase shift circuit.
However, high frequency oscillation circuits, particularly those having an oscillation frequency band of 300 MHz or more, undergo a great influence of the phase shift rate of the amplifier and the phase shift rate of wiring that connects other elements besides the phase shift requirement of the resonator and phase shift circuit.
The phase shift rate and the size of the phase shift circuit are correlated. The phase shift circuit may have a problematically large size in association with the required phase shift rate in order to satisfy the phase shift requirement of the oscillation circuit. Larger scale circuits manifest larger deviations in products.
Furthermore, the feedback loop has increased loss and needs to use an amplifier with a larger gain G to satisfy the oscillation requirement, causing influential noise problem.
Required phase shift rates are significantly different between the oscillation circuits if one uses a resonator with a resonation frequency of 155 MHz and the other with 622 MHz. Therefore, a circuit board should be designed for each resonator.
The purpose of the present invention is to provide an oscillation circuit and an electronic device that can easily satisfy the phase requirement of the oscillation loop for a desired oscillation frequency.
To solve the above problems, the present invention provides an oscillation circuit comprising a differential amplifier having plural output terminals that provide output signals having different phases from each other, a piezoelectric resonator, and a phase shift circuit for transmitting output signals that result from a predetermined phase shift of input signals, wherein the differential amplifier, piezoelectric resonator, and phase shift circuit form a positive feedback oscillation loop, characterized in that a signal selection part for selecting one of the output terminals of the differential amplifier is provided so that the positive feedback oscillation loop is completed with one of the output terminals of the differential amplifier.
In the structure above, for forming the positive feedback oscillation loop by the differential amplifier, piezoelectric resonator, and phase shift circuit, the signal selection part selects one of the output terminals of the differential amplifier to complete the positive feedback oscillation loop.
The present invention further provides an oscillation circuit comprising a differential amplifier having plural output terminals that provide output signals having different phases from each other, a piezoelectric filter, and a phase shift circuit for transmitting output signals that result from a predetermined phase shift of input signals, wherein the differential amplifier, piezoelectric filter, and phase shift circuit forming a positive feedback oscillation loop, characterized in that a signal selection part for selecting one of the plural output terminals of the differential amplifier is provided so that the positive feedback oscillation loop is completed with one of the output terminals of the differential amplifier.
In the structure above, for forming the positive feedback oscillation loop by the differential amplifier, piezoelectric resonator, and phase shift circuit, the signal selection part selects one of the output terminals of the differential amplifier to complete the positive feedback oscillation loop.
In these cases, the differential amplifier can be a differential amplifier having an ECL line receiver. The differential amplifier can have an inverting input terminal and a non-inverting input terminal. Then, the inverting and non-inverting input terminals are connected via an impedance circuit. A bias voltage is applied to one of the inverting and non-inverting input terminals and the other serves as the input terminal of the positive feedback oscillation loop.
The impedance circuit can be a tank circuit having an inductor and a capacitor. Then, the tank circuit selectively passes a desired frequency band of the output signals from the phase shift circuit. Furthermore, the piezoelectric resonator can be a quartz crystal AT cut resonator. Then, the tank circuit selectively passes an odd order overtone oscillation frequency band or desired frequency band of the output signal from the quartz crystal AT cut resonator.
A frequency selection part can be provided that selectively passes certain frequency band components of the output signals from the phase shift circuit. Furthermore, the frequency selection part can have a frequency selection condenser and a frequency selection coil that are connected in parallel. Furthermore, the frequency selection condenser has a variable capacitance to selectively pass the frequency band components of the output signals from the phase shift circuit. Furthermore, the frequency selection condenser can be a condenser that can be laser trimmed. Furthermore, the frequency selection condenser can be a condenser that is patterned on a board and can be laser trimmed. Furthermore, a frequency selection part is provided that selectively blocks frequency band components of the output signals from the phase shift circuit.
The frequency selection part can consist of a variable capacitance condenser provided in the phase shift circuit. The variable capacitance condenser can also be a condenser that can be laser trimmed. Furthermore, the variable capacitance condenser can be a condenser that is patterned on a board and can be laser trimmed. Furthermore, an output buffer circuit having an output differential amplifier can be provided on the output terminal side of the differential amplifier.
Furthermore, the output differential amplifier can be a differential amplifier having an ECL line receiver. Furthermore, a feedback buffer circuit having a feedback differential amplifier with plural output terminals that provide output signals having different phases from each other can be interposed in the positive feedback oscillation loop. Furthermore, the feedback buffer circuit can have a plural number of the feedback differential amplifiers that have the plural output terminals and is connected in parallel. Then, one of the plural feedback output terminals differential amplifiers is interposed in the positive feedback oscillation loop. Furthermore, the feedback buffer circuit can have a plural number of the feedback differential amplifiers that are connected in series. Then, one or a series of feedback differential amplifiers are interposed in the positive feedback oscillation loop.
Furthermore, the feedback differential amplifier can have irregular phase shifts in output signals.
The feedback differential amplifier can also have regular phase shifts for output signals. The feedback differential amplifier can be a differential amplifier having an ECL line receiver. Furthermore, the signal selection part performs the selection for less phase shift at the phase shift circuit. Furthermore, the phase shift circuit can be a voltage controlled phase shift circuit in which the phase shift rate is varied by applied control voltage. The piezoelectric resonator can be a SAW resonator. The SAW resonator can be made of any one of quartz crystal, langasite, and LBO (Lithium Tetraborate). The piezoelectric resonator can be a quartz crystal AT cut resonator. An electronic device can be provided with the oscillation circuit described above.